1. Field of the Invention
The present invention is related to a thin film transistor used for various image display devices and to a method for manufacturing the thin film transistor. Further, the present invention is related to a display using the thin film transistor.
2. Description of the Related Art
Conventionally, using a transistor which uses a semiconductor for a substrate and IC technology as a basis, a thin film transistor (TFT) having amorphous silicon (a-Si) or poly silicon (poly-Si) is formed on a glass substrate. Such a thin film transistor is used for a liquid crystal display, electronic paper and an electroluminescence display.
In addition, in recent years, an organic semiconductor or an oxide semiconductor has been developed. (See non-patent document 1.) That is, it was found that TFT could be manufactured at a low temperature of 200° C. or less. Therefore, a flexible display using a plastic substrate is expected. Particularly, since mobility of the oxide semiconductor is high (about 10 cm2/Vs), it is expected that a high performance TFT like poly-Si is realized. That is, it is expected that TFT having the oxide semiconductors are used not only for TFT inside a pixel but also for a circumferentially-arranged drive logic circuit.
However, in a manufactured oxide TFT, the threshold voltage of TFT was shifted when the oxide semiconductor came into contact with general resin (epoxy, acrylic or the like), wherein the shift amount was about (−30) V.
This shift phenomenon becomes a problem not only when the thin film transistor is used as a logic circuit but also when the thin film transistor is used for a display. For example, when the thin film transistor is used as a logic circuit, the thin film transistor is usually buried by an epoxy resin. However, when the oxide semiconductor comes into contact with this epoxy, the characteristics of TFT change, thereby a normal logic operation is not conducted.
In addition, when the thin film transistor is used for a display wherein an interlayer dielectric and an upper pixel electrode are formed, the oxide semiconductor comes into contact with a resin such as epoxy or acrylic which is usually used for the interlayer dielectric, thereby the characteristics of TFT change. When the thin film transistor is used for a display wherein an interlayer dielectric is not formed, the oxide semiconductor comes into contact with a liquid crystal in a case of a liquid crystal display, or the oxide semiconductor comes into contact with an adhesive in a case of electronic paper, thereby the characteristics of TFT change. In whichever case, behavior of a display device becomes abnormal.
[non-patent document] K. Nomura et al., Nature, 432, 488(2004)